1. Field of the Invention
The present invention relates to a compound semiconductor device and, more particularly, a compound semiconductor device having a mesa type bipolar transistor.
2. Description of the Prior Art
A heterojunction bipolar transistor (HBT) can be used as a high performance compound semiconductor due to its advantages of having a single power supply, a high output, and a high linearity thus, using it as a supply in a power amplifier of a mobile telephone terminal can now be expected. In addition, the HBT is used as an output device in a radio transmitting base station in which a high output, a high linearity, and a high breakdown voltage are needed.
Next, a configuration of a vertical type heterojunction bipolar transistor in the prior art will be explained with reference to FIG. 1.
In the HBT shown in FIG. 1, a buffer layer 102 made of i-AlGaAs, a subcollector layer 103 made of n.sup.+ -GaAs, a collector layer 104 made of n.sup.- -GaAs, a base layer 105 made of p.sup.+ -GaAs, and an emitter layer 106 made of n-AlGaAs are stacked in sequence on a GaAs substrate 101.
The emitter layer 106, the base layer 105, and the collector layer 104 are patterned in sequence, so that a portion of the subcollector layer 103 to which a collector electrode 107 is connected is exposed. Also, the emitter layer 106 is patterned, so that a portion of the base layer 105 to which a base electrode 108 is connected is exposed. In addition, an emitter electrode 109 is connected to the emitter layer 106.
In such HBT, in order to enhance a base-collector breakdown voltage and enhance an emitter-collector breakdown voltage on condition the base is open, a carrier density in the collector layer 104 is reduced and a film thickness of the collector layer 104 is made thick. For instance, a device in which a film thickness of the collector layer 104 is set to 2 mm or more to attain a sufficient collector breakdown voltage has been fabricated.
Such the collector layer 104 is grown by the metal organic chemical vapor deposition (MOCVD) method, the molecular beam epitaxy (MBE) method, etc. In this case, normally a growth rate is set to 0.1 to 2.0 mm/hour, so that several hours is taken to grow the collector layer 104 of 1 to 2 mm thickness. As a result, productivity has been low in the existing circumstances.
Also, in the above configuration, since a part of the emitter layer 106 to the collector layer 104 is patterned continuously by etching, a difference in level, i.e., a step existing on the subcollector layer 103 is indispensably enlarged according as the thickness of the collector layer 104 increases. If such difference in level is increased, the emitter electrode 109 extending from the emitter layer 106 is ready to be disconnected due to the step, so that the yield of the bipolar transistor is lowered.
Furthermore, in the present condition, an impurity concentration in the collector layer 104 is set low of 2.times.10.sup.16 to 5.times.10.sup.16 atoms/cm.sup.3 to enhance. However, the lower limit of the impurity concentration has been about 1.times.10.sup.16 atoms/cm.sup.3 if the impurity should be contained in the n type GaAs layer, and therefore a reproducibility of fabrication has been lost if the impurity concentration is set lower than such concentration to a enhancement of the breakdown voltage.